Recent Federal Aviation Regulations require that new transport aircraft include systems that reduce the development of flammable vapors in fuel tanks. One approach for minimizing the development of flammable vapors is to utilize an “inerting system” that flows an inert gas, such as nitrogen, into the fuel tank to reduce the oxygen concentration. The inert gas can be provided by high pressure bleed air from an engine compressor or an auxiliary power unit compressor. In either case, the high pressure air flows through a heat exchanger and then through air separation modules to separate the air into an oxygen-rich component that is exhausted overboard and an oxygen-depleted or inert gas component that flows into the fuel tank.
Another approach for reducing the development of flammable vapors in a fuel tank is to maintain the fuel tank at a relatively low temperature that is not conducive to the formation of such vapors. One method for doing so involves using an air conditioning system to displace warm air surrounding the fuel tank. Another method involves placing a cooling element, such as a cooling coil, inside the fuel tank to condense fuel vapors that may exist in the space above the fuel, i.e., in the ullage.
One shortcoming associated with existing systems for minimizing flammable vapors is that they typically operate on a full-time basis during the entire flight and all ground operations. The Federal Aviation Regulations, however, do not require full-time operation for compliance with the new rule. For example, selective use of fuel tank inerting complies with the rule if it can be shown that the probability of fuel tank flammability is significantly lower under certain conditions or during a particular phase of flight.